It is known to remove oxides of nitrogen (NO.sub.x) pollutants from exhaust streams or other gaseous emission sources which contain excess oxygen by adding NH.sub.3 to a gas containing NO.sub.x and passing the mixture over a catalyst to form non-polluting N.sub.2 and H.sub.2 O.
A wide variety of catalysts have been proposed for this purpose. See, for example, U.S. Pat. Nos. 3,279,884 and 4,115,516. Vanadium pentoxide or other oxide supported on alumina has been extensively used as an SCR catalyst. Cerium oxide has also been proposed. See, for instance, the above-mentioned U.S. Pat. No. 4,115,516 as well as U.S. Pat. No. 3,885,019.
The ability to abate NO.sub.x emissions effectively is limited by the selectivity of the catalyst for reacting NH.sub.3 with NO.sub.x rather than with O.sub.2. While prior catalysts, particularly vanadium pentoxide, have a reasonably good degree of selectivity, there is still a need to improve on the selectivity of such catalysts.
The purpose of the present invention is to provide a vanadium-based SCR catalyst which includes ceria and which shows considerably increased selectivity and higher activity than conventional vanadium pentoxide or ceria oxide-based SCR catalysts.